Depends on your build, system configuration, etc. I worked at a place where the data sci’s had a remarkable talent to build docker images based on python images they found in random places and you could wind up with charsets you wouldn’t expect. Managing the problem in code didn’t entirely work because if some third party lib print()ed something that had characters not in the charset it would crash.
I understand what you're saying, yet I've tried running on Linux
It was the interpreter's default for reading Python source code files since 3.0 (https://peps.python.org/pep-3120/), but not the default for built-in and standard library functionality. But it may have been hard to notice on many systems.
On Linux it was, but not on Windows.
Even on Linux it was not. Python would use the locale’s encoding, which is not necessarily UTF8. The POSIX locale was special cased to enable UTF8 mode tho.